It depends on what is connected to the Mini Pro. If that does not need a lot of current, the USB power is good to power the Mini Pro and the USB-ttl-serial converter.

Normally a USB-ttl-serial converter has a 5V output from the USB cable. You can use it to power the Arduino.To be sure, you could give a link to it or upload a photo.

The USB of the laptop could perhaps be less than 5V, if it doesn't work, you might have to measure the voltage. The USB it is current protected, so things should not burn if something is connected wrong.

Because of the 5V coming from the external source and the USB port, both are connected together through the pins of the Arduino.I'm using an onboard DC/DC converter to power the Arduino (not seen on the picture, it's under the upper panel).